The invention relates to shaping bond wires, particularly Free-standing bond wires used as core elements for composite interconnection element such as are described in commonly-owned, copending U.S. patent application Ser. No. 08/452,255 filed May 26, 1995 (status: pending) and its counterpart PCT patent application number PCT/US95/14909 filed Nov. 13, 1995.
Electronic components, particularly microelectronic components such as semiconductor devices (chips), often have a plurality of terminals (also referred to as bond pads, electrodes, or conductive areas). In order to assemble such devices into a useful system (or subsystem), a number of individual devices must be electrically interconnected with one another, typically through the intermediary of a printed circuit (or wiring) board (PCB, PWB).
Generally, interconnections between electronic components can be classified into the two broad categories of xe2x80x9crelatively permanentxe2x80x9d and xe2x80x9creadily demountablexe2x80x9d.
An example of a xe2x80x9crelatively permanentxe2x80x9d connection is a solder joint. Once two electronic components are soldered to one another, a process of unsoldering must be used to separate the components. A wire bond is another example of a xe2x80x9crelatively permanentxe2x80x9d connection.
An example of a xe2x80x9creadily demountablexe2x80x9d connection is rigid pins of one electronic component being received by resilient socket elements of another electronic component. The socket elements exert a contact force (pressure) on the pins in an amount sufficient to ensure a reliable electrical connection therebetween. Interconnection elements intended to make pressure contact with an electronic component are referred to herein as xe2x80x9cspringsxe2x80x9d or xe2x80x9cspring elementsxe2x80x9d or xe2x80x9cspring contactsxe2x80x9d.
Prior art techniques for making spring contact elements generally involve stamping (punching) or etching a xe2x80x9cmonolithicxe2x80x9d spring material, such as phosphor bronze or beryllium copper or steel or a nickel-iron-cobalt (e.g., kovar) alloy, to form individual spring elements, shaping the spring elements to have a spring shape (e.g., arcuate, etc.) plating the spring elements with a good contact material (e.g., a noble metal such as gold, which will exhibit low contact resistance when contacting a like material), and molding a plurality of such shaped, plated spring elements into a linear, a peripheral or an array pattern. When plating gold onto the aforementioned materials, sometimes a thin (for example, 30-50 microinches, barrier layer of nickel is appropriate.
Generally, a certain minimum contact force is desired to effect reliable pressure contact to electronic components (e.g., to terminals on electronic components). For example, a contact (load) force of approximately 15 grams (including as little as 2 grams or less and as much as 150 grams or more, per contact) may be desired to ensure that a reliable electrical connection is made to a terminal of an electronic component which may be contaminated with films on its surface, or which has corrosion or oxidation products on its surface. The minimum contact force required of each spring element typically demands either that the yield strength of the spring material or that the size of the spring element are increased. However, generally, the higher the yield strength of a material, the more difficult it will be to work with (e.g., punch, bend, etc.). And the desire to make springs smaller essentially rules out making them larger in cross-section.
In commonly-owned, copending U.S. patent application Ser. No. 08/452,255 filed May 26, 1995 (and its counterpart PCT/US95/14909 filed Nov. 13, 1995), techniques are described for shaping elongate core elements (wire stems) of composite interconnection elements using a wirebonder. A one end of a supply wire is ball-bonded to an area (e.g., terminal) on a substrate (e.g., electronic component) by urging a capillary of a wirebonder downward (z-axis) onto the substrate. The capillary is then withdrawn (upward), and as the wire plays (feeds) out of the capillary, the substrate is moved in the x-y plane to impart a desired spring shape to the portion of the wire between the substrate and the capillary. The wire is then severed adjacent the capillary, resulting in a free-standing wire stem which is mounted to the substrate. The possibility of using external, mechanical instrumentalities to impart the desired shape to the wire stem is discussed, and is elaborated upon herein.
Attention is also directed to U.S. Pat. No. 5,045,975 issued Sep. 3, 1991, entitled THREE-DIMENSIONALLY INTERCONNECTED MODULE ASSEMBLY, which discloses ball bonding a plurality of gold wires (leads) onto and substantially perpendicular to an integrated circuit die, and inserting the gold leads into plated through holes of printed circuit boards to effect an electrical and mechanical connection therebetween. The technique is also useful for interconnecting sandwiched assemblies of circuit boards. This patent illustrates the feasibility of adding a notching mechanism to a wirebonder (ball bonder), and also illustrates the technique
It is an object of the invention to provide an improved technique for shaping core elements of a composite interconnection element to have an appropriate spring shape.
It is another an object of the invention to provide a technique for fabricating resilient interconnection elements (contact structures) for electronic components, particularly for microelectronic components.
It is another object of the invention to provide resilient contact structures (interconnection elements) that are suitable for making pressure contact to electronic components.
It is another object of the invention to provide a technique for securely anchoring interconnection elements to electronic components.
According to the invention, an external mechanical instrumentality (shaping tool) is used to impart a desired shape to portion of an elongate element (e.g., a bond wire).
In an embodiment of the invention, the shaping tool is a one-part tool, which urges (pushes) against a portion of the bond wire extending between an area (e.g., terminal) on a substrate (e.g., electronic component, sacrificial substrate, etc.) and a capillary of a wirebonder.
In another embodiment of the invention, the shaping tool is a two-part tool, comprising an anvil and a die. The anvil and die are brought together, with the elongate element (e.g., wire) therebetween, to impart the desired shape to the elongate element.
According to an aspect of the invention, the shaping tool is provided with a feature which can nick or completely sever the elongate element while performing shaping.
According to an aspect of the invention, the shaping tool can be biased (at an electrical potential, including ground) to control a spark (electrical discharge) which is used to sever the elongate element.
According to another aspect of the invention, regions of reduced diameter formed by the shaping tool can xe2x80x9cattractxe2x80x9d a spark during spark-severing (e.g., from an EFO electrode).
The present invention is particularly useful for, but is not limited to, shaping core elements of composite interconnection elements, which are fabricated by bonding a one end of a bond wire to a terminal on an electronic component (or to an area of a sacrificial substrate), imparting a spring shape to the wire, and severing the wire to be a free-standing wire stem (core element). The free-standing wire stem is overcoated with one or more layers of material to impart a desired resiliency and, optionally, electrical contact characteristics to the resulting composite interconnection element. It is within the scope of this invention that the core element is other than a wire having a circular cross-section. For example, the core element may have a rectangular cross-section and be in the form of a xe2x80x9cribbonxe2x80x9d.
The disclosed techniques overcome problems associated with making spring elements of extremely small size, yet which are capable of exerting contact forces of sufficient magnitude to ensure reliable interconnections. The disclosed techniques also overcome problems associated with mounting spring contacts directly on various electronic components, such as semiconductor devices.
The use of the term xe2x80x9ccompositexe2x80x9d, throughout the description set forth herein, is consistent with a xe2x80x98genericxe2x80x99 meaning of the term (e.g., formed of two or more elements), and is not to be confused with any usage of the term xe2x80x9ccompositexe2x80x9d in other fields of endeavor, for example, as it may be applied to materials such as glass, carbon or other fibers supported in a matrix of resin or the like.
As used herein, the term xe2x80x9cspring shapexe2x80x9d refers to virtually any shape of an elongate element which will exhibit elastic (restorative) movement of an end (tip) of the elongate element with respect to a force applied to the tip. This includes elongate elements shaped to have one or more bends, as well as substantially straight elongate elements.
As used herein, the terms xe2x80x9ccontact areaxe2x80x9d, xe2x80x9cterminalxe2x80x9d, xe2x80x9cpadxe2x80x9d, and the like refer to any conductive area on any electronic component (including passive substrates and sacrificial substrates) to which an interconnection element is mounted or makes contact.
Generally, the core is a xe2x80x9csoftxe2x80x9d material having a relatively low yield strength, and is overcoated with a xe2x80x9chardxe2x80x9d material having a relatively high yield strength. For example, a soft material such as a gold wire is attached (e.g., by wire bonding) to a terminal of an electronic component, and is overcoated (e.g., by electrochemical plating) with a hard material such nickel and its alloys.
Generally, throughout the description set forth herein, the term xe2x80x9cplatingxe2x80x9d is used as exemplary of a number of techniques for overcoating the core element. It is within the scope of this invention that the core element can be overcoated by any suitable technique including, but not limited to: various processes involving deposit on of materials out of aqueous solutions; electrolytic plating; electroless plating; chemical vapor deposition (CVD); physical vapor deposition (PVD); processes causing the deposition of materials through induced disintegration of liquid or solid precursors; and the like, all of these techniques for depositing materials being generally well known.
Generally, for overcoating the core element with a metallic material such as nickel, electrochemical processes are preferred, especially electrolytic plating.
Representative materials, both for the core element and for the overcoating, are disclosed.
In the main hereinafter, techniques involving beginning with a relatively soft (low yield strength) core element, which is generally of very small dimension (e.g., 3.0 mil or less) are described. Soft materials, such as gold, which attach easily to the metallization (e.g., aluminum) of semiconductor devices, generally lack sufficient resiliency to function as springs. (Such soft, metallic materials exhibit primarily plastic, rather than elastic deformation.) Other soft materials which may attach easily to semiconductor devices and possess appropriate resiliency are often electrically non-conductive, as in the case of most elastomeric materials. In either case, desired structural and electrical characteristics can be imparted to the resulting composite interconnection element by the overcoating applied over the core. The resulting composite interconnection element can be made very small, yet can exhibit appropriate contact forces. Moreover, a plurality of such composite interconnection elements can be arranged at a fine pitch (e.g., 10 mils), even though the have a length (e.g., 100 mils), which is much greater than the distance to a neighboring composite interconnection element (the distance between neighboring interconnection elements being termed xe2x80x9cpitchxe2x80x9d).
It is within the scope of this invention that composite interconnection elements can be fabricated on a microminiature scale, for example as xe2x80x9cmicrosspringsxe2x80x9d for connectors and sockets, having cross-sectional dimensions on the order of twenty-five microns (xcexcm), or less. This ability to manufacture reliable interconnection having dimensions measured in microns, rather than mils, squarely addresses the evolving needs or existing interconnection technology and future area array technology.
The composite interconnection elements of the invention exhibit superior electrical characteristics, including electrical conductivity, solderability and low contact resistance. In many cases, deflection of the interconnection element in response to applied contact forces results in a xe2x80x9cwipingxe2x80x9d contact, which helps ensure that a reliable contact is made.
An additional advantage of the invention is that connections made with the interconnection elements of the invention are readily demountable. Soldering, to effect the interconnection to a terminal of an electronic component is optional, but is generally not preferred at a system level.
According to an aspect of the invention, interconnection elements can be pre-fabricated as individual units, for later attachment to electronic components. Various techniques for accomplishing this objective are set forth herein. Although not specifically covered in this document, it is deemed to be relatively straightforward to fabricate a machine that will handle the mounting of a plurality of individual interconnection elements to a substrate or, alternatively, suspending a plurality of individual interconnection elements in an elastomer, or on a support substrate.
It should clearly be understood that the composite interconnection element of the invention differs dramatically from interconnection elements of the prior art which have been coated to enhance their electrical conductivity characteristics or to enhance their resistance to corrosion.
In addition to controlling the resiliency of the resulting composite interconnection element, the overcoating substantially enhances anchoring of the interconnection element to a terminal of an electronic component. Stresses (contact forces) are directed to portions of the interconnection elements which are specifically intended to absorb the stresses.
One advantage of the invention is that the processes described herein are well-suited to xe2x80x9cpre-fabricatingxe2x80x9d interconnection elements, particularly resilient interconnection elements, such as on a sacrificial member, then later mounting the interconnection elements to an electronic component. In contrast to fabricating the interconnection elements directly on the electronic component, this allows for reduced cycle time in processing the electronic components. Additionally, yield issues which may be associated with the fabrication of the interconnection elements are thus disassociated from the electronic component. For example, it would be disingenuous for an otherwise perfectly good, relatively expensive integrated circuit device to be ruined by glitches in the process of fabricating interconnection elements mounted thereto. The mounting of pre-fabricated interconnection elements to electronic components is relatively straightforward, as is evident from the description set forth hereinbelow.
It should also be appreciated that the invention provides essentially a new technique for making spring structures. Generally, the operative structure of the resulting spring is a product of plating, rather than of bending and shaping. This opens the door to using a wide variety of materials to establish the spring shape, and a variety of xe2x80x9cfriendlyxe2x80x9d processes for attaching the xe2x80x9cfalseworkxe2x80x9d of the core to electronic components. The overcoating functions as a superstructure over the xe2x80x9cfalseworkxe2x80x9d of the core, both of which terms have their origins in the field of civil engineering.
One of the significant advantages of using a readily-deformable, malleable, compliant material for the wire stem is that it is readily configured to establish a shape for the overcoat applied thereto, without significantly altering the physical properties (e.g., tensile strength, resiliency, etc.) of the resulting resilient contact structure. Inasmuch as the wire stem serves as an important first step in the overall process (begun, but not completed) of fabricating a resulting contact structure, the wire stem can be characterized as an xe2x80x9cinchoatexe2x80x9d contact structure.
The composite interconnection (spring) elements of the invention are applicable to a broad range of applications, for example, for use in interposers. The subject of using composite interconnection elements in interposers has been discussed in the PARENT CASE. Generally, as used herein, an xe2x80x9cinterposerxe2x80x9d is a substrate having contacts on two opposite surfaces thereof, disposed between two electronic components to interconnect the two electronic components. Often, it is desirable that the interposer permit at least one of the two interconnected electronic components to be removed (e.g., for replacement, upgrading, and the like).
The invention differs dramatically from the prior art in that an overcoat is used to impart desires mechanical characteristics (e.g., elasticity) to an otherwise non-elastic, readily-formed, inchoate interconnection element (contact structure). In the prior art, coatings (including gold platings) are principally used to enhance electrical characteristics of interconnection elements, and to prevent corrosion thereof.
The composite interconnection elements can either be fabricated xe2x80x9cin-situxe2x80x9d on electronic components, or xe2x80x9cpre-fabricatedxe2x80x9d for later mounting to electronic components.
Among the advantages of using a shaping tool to impart a desired shape to a portion of an elongate element (e.g., wire), versus imparting the shape by moving the component relative to the capillary (or vice-versa), are:
problems with springback are largely avoided;
the desired shape can be developed more quickly;
a plurality of shaped portions of elongate elements can be produced in a more reproducible manner;
more positive control over the shape of the shaped portions of the elongate elements can be achieved; and
a plurality of free-standing, shaped wires can be mounted closer to one another on a substrate.
In instances wherein it is desired to shape and mount free-standing elongate elements to substrates, each free-standing elongate element having its own orientation, the shaping tool can be provided with sufficient degrees of freedom to accommodate the various orientations.
For shaping relatively hard materials, such as may be used for monolithic interconnection elements or hard cores of composite interconnection elements, a shaping tool may be particularly useful, and may be preferred.
Other objects, features and advantages of the invention will become apparent in light of the following description thereof.